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Original remains are the most valuable and rarest of all types of fossils. They are preserved by encasement, freezing and drying. For instance, an insect trapped in amber would be an example of an original remain preserved through encasement.

Replaced remains are more common and occur when the part of the plant or animal encased in sediment changes over time and is replaced by minerals such as petrified wood. In this process, water soaks into the encased object and the minerals in the water replace the minerals in the hard tissue. Closely related to this is the process of permineralization in which minerals from water fill in the spaces in organic tissue forming an internal cast of the organism.

In some cases, the fossils undergo heat and pressure, turning them into a carbon residue in the shape of the original object. This process is called carbonization and usually occurs in soft tissues of animals and the leaves of plants. Other fossil types are molds and casts when sediment or mud hardens around the remains to make an impression or copy of the object.